As such a storage system, a storage system disclosed in Patent Citation 1 is known, for example. According to the Patent Citation 1, in the storage system, when power supply from a primary power source to a volatile cache memory is stopped, a battery supplies power to the volatile cache memory and, meanwhile, data stored in the volatile cache memory is copied to a non-volatile memory.    [Patent Citation 1]    Japanese Patent Application Laid-Open Publication No. 2008-108026